This invention relates in general to the production of expanded metal honeycomb sheet material and, more specifically, to a method and apparatus for expanding and forming contoured metal honeycomb sheets.
Honeycomb core panels have come into widespread use in a variety of applications that require a combination of high strength and light weight. Honeycomb sheets are formed from preforms that basically consist of a large number of narrow strips of metal material, such as aluminum, stacked and bonded together in small spaced areas, such as by adhesive or solder bonding, so that when the outside edge strips are pulled away from each other the preform expands into a sheet having a uniform pattern of a very large number of small hexagonal cells, resembling a honeycomb. Typical of the prior art methods of producing honeycomb core preforms is that described by Hartsell in U.S. Pat. No. 3,077,223. This core is conventionally formed into flat panels by stretching strips on opposite sides away from each other, as described by Steele et al in U.S. Pat. No. 2,674,295.
When face sheets, such as thin aluminum sheets, are bonded to the faces of the honeycomb core, a panel with a very high strength to weight ratio results.
These honeycomb core panels are widely used in flat or cylindrical shapes for a variety of purposes. In some cases, complex curved shapes are required. For example, in aircraft engine casings or nacelles, fuselage panels, and other aerospace applications shapes, such as simple curves and compound curves, which are basically crowned cylindrical shapes are often required.
In the past, such shapes have generally been formed by pulling a honeycomb preform into an expanded flat sheet or plank. The fully expanded sheet is then roll formed by passing the sheet through a series of forming rollers. Typical of this system is that disclosed by Curran in U.S. Pat. No. 4,054,477. This method is generally suitable only for contouring the sheet in one direction. Unfortunately, this method often mutilates or distorts cells and areas of the sheet surface, so that generally only about 50% of the shaped honeycomb sheets are acceptable for use. Attempts to roll form in two directions generally result in unacceptable cell damage. Curved honeycomb core can also be formed by stretch forming the honeycomb core or on a fixed cylindrical curved surface as shown by Chester et al in U.S. Pat. No. 3,788,117. Since stretch forming often will severely distort the honeycomb cells through compression on the concave side of the core, Chester et al attempt to assure that the entire panel thickness is stretched, avoiding compression of the inner surface. This method is not capable of forming crowned curved surfaces and may cause severe distortion if the panel does not slip uniformly on the forming surface. Panels having a honeycomb core bonded to two face sheets have been formed into complex curved shapes by hydraulic methods, such as described by Fuchs in U.S. Pat. No. 3,373,460. This method can severely distort or crush the honeycomb core, as seen in Fuchs, FIG. 3, and is not suitable for shaping of honeycomb core sheets alone, since the face sheets are necessary to provide a surface to react the hydraulic forces.
Thus, there is a continuing need for methods of forming metal honeycomb sheets into simple and complex curved shapes without damage to the honeycomb cells.